Field of the Invention
The present invention relates to a light source device that includes a semiconductor laser, a phosphor, and a dichroic mirror, and a projection type display apparatus that uses the light source device.
Description of the Related Art
In recent years, semiconductor lasers have been developed that output light with a short wavelength with high light-emitting efficiency. A phosphor is excited by output light of the semiconductor laser, and light that is wavelength-converted by the phosphor is used as a light source for a projection type display apparatus.
The phosphor may be held at a fixed place and may be illuminated with excitation light. However, when the same point on the phosphor always continues to be illuminated with the excitation light, there is a case where temperature increases and light-emitting efficiently decreases, or there is a likelihood that material deterioration will occur. For this reason, the phosphor is provided on a principal surface of a rotatable circular plate, and a light source is mostly used that is configured in such a manner that the same point on the phosphor is not illuminated in a stationary manner with the excitation light.
For example, JP-A-2012-78488 discloses a projection type display apparatus that causes output light of an excitation light source to converge using a converting lens, illuminates a phosphor plate that rotates with the output light, and leads fluorescence that is emitted by the phosphor plate, to a light modulation element.
In many projection type display apparatuses including the apparatus disclosed in JP-A-2012-78488, an optical path changing element that causes one of the excitation light and the fluorescence to pass through and causes the other to be reflected is positioned in order not only to lead the excitation light to the phosphor plate, but also to lead the fluorescence that is emitted by the phosphor plate to the light modulation element. Specifically, a dichroic mirror in the shape of a flat plate is mostly positioned between the excitation light source and the phosphor plate.
In many light source devices including the device that was disclosed in JP-A-2012-78488, because the fluorescence that is emitted by the phosphor is diverging light, although passing through the converging lens, the fluorescence notably reaches the dichroic mirror while diverging. For this reason, an incident angle of the fluorescence is varied with a place of the dichroic mirror.
FIG. 9 is a diagram for illustrating this. In the light source device for the projection type display apparatus, a phosphor 3 is provided on a principal surface of a rotation plate 2 that rotates by a motor 1. Then, an excitation light source unit 4 is provided for the phosphor 3, and a laser light source that emits an excitation light Ex with a wavelength that possibly excites the phosphor 3 and an optical lens group for shaping the excitation light Ex are included in the excitation light source unit 4. A dichroic mirror 5 and a converging lens 6 are arranged between the excitation light source unit 4 and the phosphor 3. The converging lens 6 is a lens that not only causes the excitation light Ex to converge and causes the phosphor 3 to be illuminated with the excitation light Ex, but also causes fluorescence PL that is emitted by the phosphor 3 to converge and to be transferred to the dichroic mirror 5.
The dichroic mirror 5 is a mirror that causes the excitation light Ex from the excitation light source unit 4 to reflect in the direction of the phosphor 3, but passes through the fluorescence PL from the phosphor 3. The dichroic mirror 5 results from forming a dielectric multilayer film on a transparent glass having a shape of a flat plate.
In the light source in question, the converging lens 6 is positioned in such a manner that the shaped excitation light Ex that reflected by the dichroic mirror 5 converges on the phosphor 3. However, because the fluorescence PL that is emitted by the phosphor 3 is emitted as diverging light at a wide angle, the fluorescence PL not only travels in reverse on a path for the excitation light Ex, but also travels toward the dichroic mirror 5, as light flux at a wider angle than the excitation light, by way of the converging lens 6. That is, in the fluorescence that is emitted by the phosphor 3, there is also a component that passes through the converging lens 6 along an optical path that is more outward than an optical path for the excitation light, and travels toward the dichroic mirror 5.
At this point, an incident angle of the fluorescence that is incident on the dichroic mirror 5 is studied with reference to FIGS. 9 and 10. Regarding the fluorescence PL, the principal light beam that passes along an optical axis of the converging lens, the fluorescence that passes the side that is the closest to the rotation center of the rotation plate 2, and the fluorescence that passes the side that is the remotest from the rotation center are PL0, PL1, and PL2, respectively. Incident angles at which principal light beam PL0, fluorescence PL1, and fluorescence PL2 are incident on the dichroic mirror 5 are defined as α0, α1, and α2, respectively. Moreover, the incident angle is an angle that a line normal to a mirror surface of the dichroic mirror 5 and incident fluorescence make with respect to each other. Furthermore, an angle of the fluorescence toward the dichroic mirror 5 after passing through the converging lens 6, that is, an angle that the fluorescence PL1 and fluorescence PL2 make with respect to each other is defined as 2×θ (where θ is a converging half-angle).
If a direction of the dichroic mirror 5 is set in such a manner that an incident angle (α0) of the principal light beam PL0 is 45 degrees, and a numerical aperture (NA) of the converging lens 6 is set to be 0.174, as illustrated in FIG. 10, an incident angle (α1) of the fluorescence PL1 is 35 degrees, and an incident angle (α2) of the fluorescence PL2 is 55 degrees. Because NA=sin θ, the converging half-angle θ is equivalent to 10 degrees.
In this manner, there is a difference of 20 degrees in an incident angle to the dichroic mirror 5 between the fluorescence PL1 and the fluorescence PL2. When the incident angle to the dielectric multilayer film that is formed on a surface of the dichroic mirror 5 varies with the fluorescence, an optical path within the multilayer film varies and thus a difference occurs in transmission and reflection characteristics for the fluorescence. For this reason, the nonuniformity occurs to the fluorescence that is transmitted by the dichroic mirror 5, and reduction in a light amount occurs that causes a problem to an image display apparatus.
Besides the example described above, in the case where an angle at which light that is used for illuminating the projection type display apparatus is incident on the dichroic mirror differs from place to place, the transmission and reflection characteristics becomes nonuniform, and the color irregularity or the reduction in the light amount is caused to occur.
In order to suppress the color irregularity in question, it is also considered that a so-called wedge filter is provided, but this leads to a factor that increases an extremely high cost.
Accordingly, it is demanded to realize a light source device at a low cost, by which, when divergent illumination light for illuminating a projection type display apparatus is caused to be incident on a dichroic mirror, uniformity of the transmission and reflection characteristics is secured and reduction in the light amount are suppressed.